Electrostatic powder coating method

ABSTRACT

A charged unfixable powder image is formed on an electrically conductive substrate and then a second powder having the same charge polarity as that of the unfixable powder image is electrostatically sprayed over the image bearing surface, while the unfixable powder image still retains a sufficient amount of charge, whereby the second powder is preferentially deposited on the unfixable powder-difficient regions of the image bearing surface. The second powder layer is then fixed to form a continuous film. Finally, the unfixable powder image is removed.

United States Patent 191 Iwasa et al.

' [451 Dec. 23, 1975 1 ELECTROSTATIC POWDER COATING METHOD [75] Inventors: Masaltazu lwasa; Haiime Miyatuka;

Kazuo Horikawa, all of Asaka; Tomisaku Wakabayashi, Odawara, all of Japan [73] Assignee: Fi ii Photo Film Co., Ltd.,

Minarni-ashigara, Japan [22] Filed: Feb. 28, 1974 [21] Appl. No.: 446,756

[30] Foreign Application Priority Data Mar. 5, 1973 Japan 48-25846 [52] US. Cl. 427/21; 427/22; 96/1 SD [51] Int. Cl. 603G 13/08 [58] Field of Search 117/175; 9611 R, 1 SD; 355/3 R, 3 D, 3 P, 17; 101/D1G. 13; 118/637;

[56] References Cited UNITED STATES PATENTS 2,924,519 2/1960 Bertelsen 117/17.5 3,010,842 11/1961 Ricker 1f17/37 LE 3,013,890 12/1961 Bixby 1l7/l7.5 3,038,799 6/1962 Metcalfe et a1 117/1 7.5 3,236,639 2/1966 Tomanek ll7/l7.5 3,650,797 3/1972 Tomanek ll7/l7.5 3,764,312 10/1973 Honto 1l7/l7.5 3,773,507 11/1973 Sato et al. 96/1 R 3,775,103 11/1973 Sadamatsu ct 96/1 R 3,779,748 12/1973 Mosehauser et 1 17/1 7.5 3,825,421 7/1974 Tamai 96/1 R Primary ExaminerMichael Sofocleous Attorney, Agent, or Firm-Gerald .l. Ferguson, Jr.; Joseph .1. Baker [57} ABSTRACT 18 Claims, No Drawings ELECTROSTATIC POWDER COATING METHOD BACKGROUND OF THE INVENTION selective powder paint coating layer. The present invention further relates to a method carried out on a plate bearing an unfixable powder image thereon whereby a powder paint is deposited selectively on the powder-difficient region of the image bearing surface to obtain a reversal paint powder distribution relative to the unfrxable powder image with the unfixable powder image being removed after the powder paint layer is fixed to form a continuous film.

2. Description of the Prior Art A known selective electrostatic powder coating method comprises manually covering areas of a surface which are not desired to be coated with a masking material such as paper, adhesive tape, etc. Instead of this troublesome manual covering, an automatic method utilizing a photoconductive powder as a covering material has been developed as disclosed in U.S. Pat. application Ser. No. 267,757, filed June 30, 1972 (now U.S. Pat. No. 3,833,365). The automatic method comprises the following steps:

i. A photoconductive powder is uniformly spread on a surface to be coated and charged.

ii. To form areas on the surface which are to be selectively covered, the photoconductive powder layer on the surface is discharged by a selective light exposure.

iii. The discharged photoconductive powder whose electrostatic attraction to the surface to be coated disappears is removed using an air stream, and a photoconductive powder image is formed selectively on the surface in areas where coating is not desired.

iv. An electrostatic powder coating is then applied on the powder image bearing surface using an electrostatically charged powder paint having the same charge polarity as that of the powder image, while the powder image still retains a sufficient amount of charge, whereby the powder paint particles deposit selectively on the powder-difficient areas of the surface as a result of electrostatic repulsion between the powder paint particles and the photoconductive powder particles.

v. The surface thus coated is uniformly exposed to light to dissipate the electrostatic charge of the photoconductive powder.

vi. The discharged photoconductive powder particles are then removed using an air stream.

vii. The powder paint coating layer is fused to form a continuous film.

In the above described coating method the thickness of the coating film depends on the electrostatic potential difference between the photoconductive powder region and the powder paint region. In other words, since the photoconductive powder particles and the powder paint particles have charges of the same polarity, the powder paint particles deposit selectively on the previously deposited powder paint layer when the surface electrostatic potential of the photoconductive powder layer is higher than that of the powder paint layer. Therefore, the powder paint particles continue to deposit on the previously deposited powder paint layer until the surface electrostatic potential of both the photoconductive powder layer and the powder paint layer becomes equal. Further powder coating causes uniform deposition of powder paint particles on the entire surface. Powder paint deposition of a large amount on the photoconductive powder layer requires an air stream of high speed in the step vi) described above. A high speed air stream removes not only the photoconductive powder particles but also powder paint particles of the powder paint coating layer, because the powder paint particles repulse each other as a result of electrostatic repulsion. The larger the thickness of the powder paint coating layer is, the greater the removal of the powder paint particles by the air stream. Therefore, it is clear that the thickness of the powder paint coating layer is roughly determined by the surface electrostatic potential of the photoconductive powder layer.

Further, in the step vi) above it is inevitable for a small amount of powder paint particles to be scattered, while the photoconductive powder particles are being removed by the air stream. Part of the scattered powder paint particles deposit on areas of the surface where no deposit of powder paint particles is desired, after the photoconductive powder particles are removed. Even if the photoconductive powder particles are removed using a method other than an air stream which does not scatter the powder paint particles, some powder paint particles deposit on the surface due to a change in the distribution of the lines of electric force after the photoconductive powder particles are removed. Even a small amount of the powder paint deposition on a surface to be welded, for example, where no powder paint deposition is desired becomes a drastic defect. The amount of such undesired powder paint deposition on the surface where no powder paint should be deposited increases as the thickness of the powder paint coating layer increases.

Generally speaking, a thicker powder coating layer provides more advantages than a thinner one. Therefore, it is very important to increase the upper limit of the thickness of the powder coating layer allowable to improve the practicality of a powder coating method.

SUMMARY OF THE INVENTION One object of this invention is to provide an electrostatic powder coating method in which powder paint particles are not deposited on areas of the surface where powder paint particles should not be deposited.

Another object of this invention is to provide an electrostatic powder coating method which allows the use of a thicker powder paint coating layer than before.

This invention comprises an electrostatic powder coating method which comprises forming a charged unfixable powder image on an electrically conducting substrate, electrostatically spraying a second charged powder having the same charge polarity as the polarity of the unfixable powder image over the unfixable powder image bearing surface of the substrate to form a layer of the second powder, fixing the second powder layer to form a continuous coating film and removing the unfixable powder image.

DETAILED DESCRIPTION OF THE INVENTION A main feature of the present invention is to use an unfixable powder layer as a covering material.

The present invention is carried out as follows: I. A charged unfixable powder image is formed on a surface to be coated.

3 2. An electrostatic powder coating is then applied on the powder image bearing surface using a powder paint (a second powder) having the same charge polarity as that of the unfixable powder image, while the unfixable powder image still retains a sufficient amount of 5 charge, whereby the powder paint particles are deposited in selected areas of the surface as a result of the electrostatic repulsion between the unfixable powder image and the powder paint particles.

3. The powder paint coating layer is fused to form a continuous coating film.

4. The unfixable powder particles are removed using any suitable means, such as applying an air stream, water stream or brushing, whereby a final paint coating is obtained.

The unfixable powder particles used in the present invention do not form a continuous film when heated to a temperature at which the powder paint particles are fused to form a continuous coating film. A most preferred example of an unfixable powder is a photoconductive powder comprising unfixable cores, such as glass beads, and photoconductive surface layer coated thereon.

The present invention thus provides an improvement in the electrostatic powder coating method described in US. Pat. application Ser. No. 267,757, filed June 30, 1972 mentioned above.

The prior method suffers from an undesired deposition of powder paint particles on areas of the surface where the powder paint particles should not be deposited and insufficient deposition of powder paint particles on areas of the surface where a large amount of powder paint particles should be deposited to form a thick powder coating layer.

The present invention has overcome these problems by using an unfixable powder instead of a fixable photoconductive powder which is used in the above prior art method. In the present invention the powder paint coating layer is fused, for example, by heating or with a solvent, to form a continuous coated film before the unfixable powder layer is removed using an air stream, for example, while in the prior method the powder paint layer was fused after the fixable photoconductive powder layer was removed. Therefore, deposition of neither the powder paint particles nor the unfixable powder particles occurs, because the unfixable powder is removed completely from the surface using an air stream of high speed and because no powder paint particles are present which could be scattered by an air stream in the fixed powder paint film. Also, the thickness of the powder coating layer can be increased because of the same reason.

The unfixable powder which is used in this invention must be removed by physical means after the powder paint coating layer is converted into a continuous coating film. The unfixable powder also must electrostatically chargable, and a desirable particle size for the unfixable powder can be that conventionally employed, generally ranging from several microns to several thousand microns. Also, a narrow particle size distribution and spherical or semi-spherical shape are also desirable for the unfixable powder.

The unfixable powder image can be formed on the surface suitably by sprinkling, coating, pressing, transferring or spraying the unfixable powder particles in an image-wise fashion. An electrophotographic imaging method can be advantageously utilized in the present invention.

In using an electrophotographic imaging method, a powder with a particle size ranging from about 20 to 300 preferably with 60% having a particle size within :20 of the average particle size and having photoconductive surface coating layers with a thickness of about 0.8 to l5 microns, preferably. 2 to 5 microns on transparent and spherical cores having a particle size ranging from about 15 to 300 microns is especially suitable as an unfixable photoconductive powder. This composition is quite different from that of the photoconductive powder used in the prior method. Formerly the photoconductive powder used was provided with a very high degree of fixability in contrast with that of the present invention. That is, fixability was provided using a spherical thermoplastic polymer obtained using a suspension polymerization method, for example, using acrylic esters as a spherical core which occupied a major part of the composition. A photoconductive powder image was fused by heating, for example, to form a continuous film.

On the contrary, the core material for the photoconductive powder used in the present invention desirably has a softening temperature higher than the temperature which is applied during the powder coating process which usually is conducted at a temperature ranging from about 150C to about 300C. Typical core materials include glass beads, glass powder, flint shot, finely divided quartz powder, sintered alumina and magnesia. In particular, many kinds of glass beads with particle sizes ranging from about 20 microns to microns or more are commercially available, and any of these can be chosen depending on purposes. More over, advantageously the repeatability, e.g., the ability to reuse, freedom from fatigue and ease of production, of the characteristics of a photoconductive powder is easily retained, because the shape of the glass beads is spherical. In fact, when glass beads were used as a core material in the present invention, quite good results were obtained.

The photoconductive surface layer coated on the core is mainly composed of finely-divided photoconductor particles and a resinous binder similar to a conventional photoconductive powder.

Since a thickness of several microns is large enough for the photoconductive surface layer, the photoconductive powder is not fixed as a result of softening by heating if the composition of a conventional photoconductive surface layer is used. However, if the ratio of the resinous binder is too large as compared with that of the photoconductor particles, the photoconductive layer may be fixed. That is, if the weight ratio of the resinous binder is larger than that of the photoconductor particles, fixing can occur. However, since the photoconductive powder of such a composition ratio has an extremely low light sensitivity and exhibits a residual charge, its characteristics as a photoconductive powder are extremely poor. Consequently, a photoconductive powder with a usual composition ratio is not possible to be fixed.

The weight ratio of a photoconductor powder to the resinous binder usually ranges between about 20:1 and 4:1. To increase unfixability it is desirable to increase the ratio of the photoconductor powder to the resinous binder. However, too large a ratio results in a photoconductive powder which is poor in charging or other characteristics. Therefore, more preferably the weight ratio of the photoconductor powder to a resinous binder ranges from about 10:1 to 6:l.

Suitable photoconductor powders for the photoconductive surface layer are inorganic such metal oxides, metal sulfides, metal selenides and metal tellurides of metals such as zinc, cadmium, copper, lead and titanium, e.g., zinc oxide, titanium dioxide, cadmium sulfide, copper oxide, lead oxide, a mixture of cadmium sulfide and cadmium carbonate, etc., or organic photoconductors such as polyvinylcarbazole, bromo polyvinylcarbazole, phthalocyanine metal complexes, indolines, anthracenes, etc. can be used.

As the resinous binder almost all resinous binders used for conventional binder type electrophotographic light sensitive materials such as Electrofax (trade name, a rosin modified alkyd resin produced by Japan Reichold Co.) can be used. That is, alkyd resins, styrene or acrylic ester modified alkyd resins, rosin and/or phenol resin modified alkyd resins, epoxy ester resins, terpene resins, butylated melamine resins, styrene copolymers (e.g., styrene copolymerized with copolymerizable monomers such as butadiene, acrylonitrile, acrylic acid esters and methacrylic acid esters, etc.), vinyl chloride-vinyl acetate copolymers, polyvinyl acetate, partially saponified vinyl chloride-vinyl acetate copolymers, vinyl acetate copolymers (e.g., vinyl acetate copolymerized with copolymerizable monomers such as crotonic acid, acrylic acid, methacrylic acid, acrylic acid esters and methacrylic acid esters), polyalkyl methacrylates, polyalkyl acrylates and copolymer mainly consisting of alkylacrylates or alkylmethacrylates can be used. A suitable molecular weight for these resins can range from about 2000 to 500,000.

The coating liquid of the light sensitive material is prepared mainly using a medium, such as toluene, xylene, styrene, butyl acetate, amyl acetate, etc., a finelydivided photoconductor powder and a resinous binder as described above which is then mixed with the core material. The mixture is then, spray dried, for example, to obtain a photoconductive powder. Alternatively, crushing the welLblended mixture of the photoconductor and the binder, precipitating the photoconductorbinder mixture from a binder solution or suspension, vacuum deposition and the like can be employed. in the prior method an acrylic resin, for example, was used as a core material for a photoconductive powder, consequently the solvent resistance of such a photoconductive powder was poor. On the contrary, in the present invention a core material having a high softening point and a high solvent resistance such as glass beads which are perfectly solvent resistant is used. Therefore, a broad range of compositions for the photoconductive surface layer can be selected independent of solvents. In a specific embodiment where a photoconductive powder was prepared using a copolymer of vinyl chloride and vinyl acetate as a resinous binder and a zinc oxide powder as a photoconductor powder, the unfixability of the photoconductive powder was increased greatly, because the zinc oxide powder acted as a catalyst for the thermal decomposition of the binder.

Next, a powder paint to be electrostatically coated is described. In this specification the term "powder paint" is used for convenience to distinguish it from the unfixable powder. The necessary requirements for the powder paint which is to be applied on the unfixable powder image are that it have the ability to retain an electrostatic charge for a pre-determined period and the ability to fuse at a predetermined temperature to form a continuous film. In general, powder paints comprise pigments such as chrome yellow, ultramarine, red iron oxide, calcium carbonate, carbon black, vinyl monomer graft carbon black, titanium dioxide, etc., dispersed in a resinous material and have a particle size ranging from about 3 microns to 5 mm, preferably 20 to microns or so. As resinous materials epoxy resins, polyamides, polyesters, polyvinyl chlorides, cellulose acetate butylates, acryl resins and methacryl resins are included. Powder paints and their technology are well known in the art, for example, as described in; (l A. B. Zimmerman, G. Kappas, Paint and Varnish Production, 55 (2), p. 57 (1965), (2) K. M. Oesterle, I. Szasz, Journal of the Oil and Color Chemists Association, 48 (10), p. 956 (1965), (3) Shell Chemical Paint Technology News, No. 69, (4) T, Kubota, Coating Technology, 67 (I4), p. 8 (1962) and (5) British Pat. No. 1,040,897. Also, resinous materials which can be hardened upon reaction with polyhydric alcohols, alkylene isocyanates, arylene isocyanates and so on or hardened upon irradiation of electromagnetic waves or corpuscular beams are used as resinous materials for powder paints. In this case resinous materials having reactive groups such as epoxy rings, hydroxyl groups, a.B-unsaturated acryloxy groups (e.g., acryloxy groups, methacryloxy groups, cinnamoyloxy groups and so on). allyl groups, cinnamyl groups, quinone azide groups and sulfonyl azide groups can be used, and solvent resistance, scratch resistance and so on can be improved greatly by applying post treatment.

Any substrate can be used for the present invention provided that it has a suitable surface electrical conductivity and that it does not exhibit a strong adhesivity to a finelydivided powder. A preferred range of conductivity is not less than 10" (ohm squareY. Typical materials include metallic plates such as iron or aluminum, paper or paint coated steel plates treated with electrically conductive materials such as alumina, calcium carbonate, magnesia, etc., coated using a resin such as styrene-maleic acid anhydride, polyvinyl alcohol, etc.

To remove the unfixable powder image any means which does not remove the continuous coating film can be conveniently used. Usual means such as applying an air or liquid stream or mechanical force can be used.

The present invention has the following advantages over the prior method described in US. Pat. application Ser. No. 267,757, filed June 30, l972.

One advantage of the present invention is that no powder paint deposition occurs on the surface where no powder paint deposition is desired. No powder paint deposits on the surface where no powder paint deposition is desired, because the unfixable powder layer exists on the surface until the continuous coating film is formed. This fact is very advantageous in case a material, which includes a portion to be welded, is powder coated.

Another advantage of the present invention-is in the ability to use a thicker coating layer than previously possible. In the prior method which used a fixable photoconductive powder, a thick coating film was impossible to obtain because the powder paint deposited on the surface where no powder paint deposition was desired during or after the photoconductive powder was removed. However, because an unfixable powder is used in the present invention, removal of the unfixable powder before a continuous film is formed is not nec essary. The refore, even if a small amount of powder paint deposits on the surface of the unfixable powder image,

it is removed together with the unfixable powder since the powder paint on the unfixable powder image does not form a continuous film when heating is applied. Consequently, the coating of the powder paint can be continued until a small amount of the powder paint deposits on the unfixable powder images, and as a result, a thick coating film can be obtained. A suitable coating thickness for the unfixable powder ranges from about 40 to 800 1., preferably lOO to 400p and for the powder paint from about 50;; to mm, preferably I00 to 1000 In the following, specific examples are given to facilitate a better understanding of the invention and the results obtained. Unless otherwise indicated, all parts. percents, ratios and the like are by weight.

EXAMPLE 1 Glass beads of particle sizes from 44 to 88 microns (Toshiba Glass Beads GB 733, trade name, a product of Toshiba Electric) were used as a core material, and on the surface of the glass beads the mixture of the following ingredients was coated to obtain an unfixable photoconductive powder with a surface coating layer of three microns.

methanol solution) A spray drying method was used to coat the glass beads with the above mixture.

The photoconductive powder thus obtained was sprinkled on an aluminum plate having a thickness of 2mm at a density of lg/m and charged, and then exposed to a light image. The charging was done using a negative corona discharge with a corona voltage of 60KV and a corona electrode 2cm above the aluminum plate. The electrostatic latent image thus obtained was developed by applying an air stream (at l8m/sec onto the aluminum plate) and a powder image was formed.

After the powder image was charged again using the same conditions as described above to recover the dark decay, a powder paint (a polyester based paint, Evaron 4000, trade name, product of Chugoku Toryo Co., Ltd.) was sprayed over the aluminum plate.

The electrostatic coating was continued until a thin layer of about 100 to 200p (in a powder condition) of the powder paint was formed on the powder image. The plate was then stored at a constant temperature of 200C for 7 minutes to form a continuous coating film. After the plate was cooled, the photoconductive powder image was brushed away using a nylon brush with brush elements of a diameter of 0.3 mm.

Thus, a coating film thickness of over 100 microns was obtained.

The binder,.vinyl chloride-vinyl acetate copolymer, of the photoconductive powder of this example was thermally decomposed at the baking temperature of the powder paint ranging from [80 to 250C. Of

8 course, the glass beads for the core material were not fused in this temperature range, therefore, the photoconductive powder did not form a continuous film and was removed easily. This fact is a special advantage of the invention as described in this example.

EXAMPLE 2 The surface of glass beads having an average particle size of 20 microns was coated with a mixture of the following ingredients using a spray drying method.

parts by weight Then the photoconductive powder was stored at C for about 15 hours and hardened. Using the photoconductive powder thus obtained, a powder paint coating was accomplished in the same way as described in Example I and a good result with a coating film thickness of over microns was obtained.

Although the surface coating layer of the photoconductive powder in this example was not decomposed at a baking temperature unlike Example I, a continuous film was not formed because the thickness of the photoconductive surface coating layer was small.

Also, no deposition of the photoconductive surface coating layer on the plate was observed.

EXAMPLE 3 The same procedure as described in Example 1 was followed except that the same amount of a mixture of cadmium sulfide-cadmium carbonate [CdS.n CdCO (nzl 4), Cadmium Yellow Orange, a product of the Mitsui Kinzoku Kogyo)] was used instead of the zinc oxide powder.

A good result almost the same as in Example 1 was obtained.

EXAMPLE 4 The same procedure as described in Example 2 was followed except that the same amount of an epoxyester resin (Epico Sol 80hr, a product of the Japan Coating Co., oil content 40%) instead of the silicone resin and 0.15 parts by weight of cobalt naphthenate (in toluene) as a hardener were used and that hardening was done at 50C for 20 hours.

A good result almost the same as in Example 2 was obtained.

EXAMPLE 5 The same procedure as described in Example 4 was followed except that the same amount of a styrene modified alkyd resin (Styresol 4250, a trade name of a product of the Japan Reichold Co., acid value less than 8) was used instead of the epoxyester resin.

A good result the same as in Example 2 was obtained.

EXAMPLE 6 The same procedure as described in Example 1 was followed except that a finely divided quartz powder having an average particles size of 50 microns was used instead of the glass beads and that the density of the sprinkled powder was about 220g/m A good result almost the same as in Example I was obtained.

EXAMPLE 7 The same procedure as described in Example 1 was followed except that the amount of the resinous binder was reduced to 70 parts from 100 parts by weight. In this example the amount of the resinous binder of the photoconductive powder was simply decreased, and as a result, the unfixability of the photoconductive powder was higher than that of Example 1, and the photoconductive powder was removed from the plate more easily than in Example 1.

EXAMPLE 8 The same procedure as described in Example 1 was followed except that glass beads having an average particle size of 40 microns were used.

A good result almost the same as in Example I was obtained.

EXAMPLE 9 The same procedure as described in Example I was followed except that an air jet stream was used to remove the photoconductive powder image after the continuous film was formed. The speed of the air jet stream was between about 30 and about 40m/sec on the surface of the aluminum plate. in this example no mechanical force was applied to the continuous film and no scratching was observed on it.

A good result was obtained.

Instead of an air jet stream a water stream with a pressure of 4.5kg/cm was used, and a good result was obtained.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein with out departing from the spirit and scope thereof.

What is claimed is:

1. An electrostatic powder coating method comprising: forming a charged unfixable powder image on an electrically conducting substrate; electrostatically spraying a second charged powder having the same charge polarity as the polarity of said unfixable powder image over the unfixable powder image bearing surface of said substrate to form a layer of said second powder; fixing said second powder layer; and removing, subsequent to the fixing of said second powder layer, said unfixable powder image.

2. The method as described in claim I, including charging said unfixable powder image again to recover a dark decay before applying said second powder.

3. The method as described in claim 1, wherein said forming of said unfixable powder image is by transferring said unfixable powder image from another surface.

4. The method as described in claim 1, wherein said unfixable powder is photoconductive.

5. The method as described in claim 4, wherein said unfixable photoconductive powder comprises a transparent core material and a photoconductive surface layer coated on said transparent core material.

6. The method as described in claim 5, wherein said core material is not softened at a temperature between about C and about 300C.

7. The method as described in claim 6, wherein said core material is glass beads, glass powder, flint shot, finely-divided quartz powder, sintered alumina or magnesia.

8. The method as described in claim 5, wherein said photoconductive surface layer comprises a finelydivided photoconductor powder and a resinous binder.

9. The method as described in claim 8, wherein the weight ratio of said photoconductor powder to said resinous binder is between 20:1 and 4:1.

10. The method as described in claim 8, wherein said photoconductor powder is selected from the group consisting of zinc oxide, titanium dioxide, cadmium sulfide, a mixture of cadmium sulfide and cadmium carbonate, polyvinylcarbazole and bromo polyvinylcarbazole.

l l. The method as described in claim 8, wherein said photoconductive surface layer comprises a photoconductive zinc oxide powder and a vinyl chloride-vinyl acetate copolymer.

12. The method as described in claim 1, wherein said fixing of said second powder layer is by heating to form a continuous film.

13. The method as described in claim I, wherein said second powder comprises a pigment in a resinous material.

14. The method as described in claim 1, wherein said removing of said unfixable powder image is by brushl5. The method as described in claim 1 wherein said removing of said unfixable powder image is by applying an air stream.

16. The method as described in claim 1 wherein said removing of said unfixable powder image is by applying a liquid stream.

17. The method as described in claim 1 wherein said removing of said unfixable powder image is by applying a mechanical force.

18. The method as described in claim 1, including continuing said spraying of said second powder until a thin layer of said second powder is formed on said unfixable powder image.

It It l I 

1. AN ELECTROSTATIC POWDER COATING METHOD COMPRISING: FORMING A CHARGED UNFIXABLE POWDER IMAGE ON AN ELECTRICALLY CONDUCTING SUBSTRATE; ELECTROSTATICALLY SPRAYING A SECOND CHARGED POWDER HAVING THE SAME CHARGE POLARITY AS THE POLARITY OF SAID UNFIXABLE POWDER IMAGE OVER THE UNFIXABLE POWDER IMAGE BEARING SURFACE OF SAID SUBSTRATE TO FORM A LAYER OF SAID SECOND POWDER; FIXING SAID SECOND POWDER LAYER; AND REMOVING, SUBSEQUENT TO THE FIXING OF SAID SECOND POWDER LAYER, SAID UNFIXABLE POWDER IMAGE.
 2. The method as described in claim 1, including charging said unfixable powder image again to recover a dark decay before applying said second powder.
 3. The method as described in claim 1, wherein said forming of said unfixable powder image is by transferring said unfixable powder image from another surface.
 4. The method as described in claim 1, wherein said unfixable powder is photoconductive.
 5. The method as described in claim 4, wherein said unfixable photoconductive powder comprises a transparent core material and a photoconductive surface layer coated on said transparent core material.
 6. The method as described in claim 5, wherein said core material is not softened at a temperature between about 150*C and about 300*C.
 7. The method as described in claim 6, wherein said core material is glass beads, glass powder, flint shot, finely-divided quartz powder, sintered alumina or magnesia.
 8. The method as described in claim 5, wherein said photoconductive surface layer comprises a finely-divided photoconductor powder and a resinous binder.
 9. The method as described in claim 8, wherein the weight ratio of said photoconductor powder to said resinous binder is between 20:1 and 4:1.
 10. The method as described in claim 8, wherein said photoconductor powder is selected from the group consisting of zinc oxide, titanium dioxide, cadmium sulfide, a mixture of cadmium sulfide and cadmium carbonate, polyvinylcarbazole and bromo polyvinylcarbazole.
 11. The method as described in claim 8, wherein said photoconductive surface layer comprises a photoconductive zinc oxide powder and a vinyl chloride-vinyl acetate copolymer.
 12. The method as described in claim 1, wherein said fixing of said second powder layer is by heating to form a continuous film.
 13. The method as described in claim 1, wherein said second powder comprises a pigment in a resinous material.
 14. The method as described in claim 1, wherein said removing of said unfixable powder image is by brushing.
 15. The method as described in claim 1 wherein said removing of said unfixable powder image is by applying an air stream.
 16. The method as described in claim 1 wherein said removing of said unfixable powder image is by applying a liquid stream.
 17. The method as described in claim 1 wherein said removing of said unfixable powder image is by applying a mechanical force.
 18. The method as described in claim 1, including continuing said spraying of said second powder until a thin layer of said second powder is formed on said unfixable powder image. 